Warzone 2100 is an old favourite of several members of this Forum, it's achieved 'permanently installed' status on my computer, and I still love playing it from time to time, partly because of the depth of the tech tree, to the point where you can install anti-artillery rader that will plot the course of incoming artillery barrages and return fire to the point of origin, never seen a game so dedicated to the art of technological warfare. In many ways, once it gets to the point of the Global Uplink, it kind of ruins the fun.
Turtling can work for a while, and the AI doesn't build very good bases, often trapping large chunks of their own units behind buildings, which can be irritating, but all in all, I still consider it one of the all time greats when it comes to RTS games.